The invention relates to a target assembly for an electron linear accelerator (LINAC). In particular, it relates to a target for a LINAC capable of supplying x-ray beams of different energies.
A LINAC supplies x-rays by directing an electron beam onto a target, where the decelerated electrons emit the desired x-ray quanta. Such a target must endure high thermal stresses for a long period of time; in a typical example 300 to 500 watts are created within a target area of 1 to 2 mm diameter for 40 minutes.
A target assembly with an improved cooling capacity is described in U.S. Pat. No. 4,323,780. In this arrangement, the target is suspended in a recess of a solid metal plate. The target divides the recess into an upper and lower chamber, each being part of a channel. In an operation, a liquid coolant is directed through the channels so that the target is at both sides directly exposed to a streaming medium. Such a system is relatively complicated; moreover, its beam conversion capability is impaired by the fact that electrons and photons must pass through additional layers of ray-absorbing and dispersing material.
Further problems arise, if a LINAC is required to supply x-ray beams with a variety of beam energies: an optium ratio between beam power and beam quantity, i.e. angular intensity distribution and energy spread, is obtained when the target is about one-fifth of the electron penetration range in thickness (see, e.g. Electro Medica, 3-4 (1977) 101, section "Roentgenbremsstrahlung"). When the target thickness exceeds this value, the beam power increases somewhat but the angular intensity distribution degrades significantly and the energy profile is broadened by low energy components. Thus, the target (which usually consists of a heavy metal like tungsten or gold) can only be optimized for one e.sup.- beam energy. To obtain optimum results at other energy levels as well, the accelerator may be operated with selected ones of a set of targets, each adjusted to a certain energy level. In the published European patent application 149571, there is taught to arrange different targets on a common support, which may be a slidable plate or a rotatable disc and may be moved together with, or independent of, the flattening filter. All these structures are elaborate--the targets must be positioned very carefully--and can conduct heat away from the target only to a limited degree.
An object of the invention is to provide a LINAC target assembly which allows different tradeoffs between beam power and quality.
A more specific object is to provide a LINAC target allowing a favorable optimum ratio between beam power and quality for different electron beam energies.
A further object is to provide a simple LINAC target assembly capable of dissipating the heat due to energy losses.
Still another object is to improve on the LINAC target assemblies in the art.